1. Field of the Invention
The present invention relates to position control of vehicles and more particularly to position control utilizing momentum devices such as control moment gyros, (CMGs) and reaction wheel assemblies (RWAs) which find major utility in the field of space satellites.
2. Description of the Prior Art
CMGs and RWAs have long been used to position and rotate space satellites through the use of angular momentum. CMGs operate to produce a relatively large torque along a first axis when a spinning mass is rotated with a smaller torque about a second axis, which process is often referred to as torque multiplication. RWAs include a spinning mass which exerts a relatively small torque along the spin axis as the mass is caused to spin faster or slower. Accordingly, combinations of CMGs (usually 3 or more in an array) are situated in non-coincidental mounting planes so that their torques may be applied, using various combinations, in any desired direction of rotation. Reaction wheel assemblies are similarly disposed with several (usually 3 or more in an array) reaction wheels aligned to cause rotation in any direction. CMGs are most commonly used where large and/or rapid motions, or high inertia equipment, needs to be moved with high precision such as in spacecraft slew maneuvers. RWAs are usually used where smaller movements are necessary, such as controlling the direction of scan of a sensor, or detector, such as small camera or radar antenna in satellites, which operate to view areas or targets on the earth. The large motions are referred to herein as "slewing" while the smaller motions are referred to herein as "scanning".
CMGs are extremely costly devices since their use normally requires great strength and precision and thus costly construction where high quality materials and sophisticated processes are involved. RWAs are considerable less costly since their design is much simpler and their uses are for smaller devices. There exists a need for a low cost device capable of rapidly rotating high inertia devices from one position to another as, for example, in a satellite which is used to scan targets on earth but which must move from one target area to another more quickly than is possible with an RWA array.
Additionally, because CMGs produce a large torque in an output axis given a small torque in the gimbal axis through torque multiplication, disturbances in the input torque also get amplified resulting in large disturbances in the output torque from the device. These disturbances can have a detrimental effect on the performance of the sensor or detector that the spacecraft is attempting to precision point during a scan. If the spacecraft inertia is sufficiently small, even the most precision CMGs can produce disturbances that will render the spacecraft unable to point the sensor within its required precision to properly operate. Conversely, RWA arrays produce relatively low disturbances when compared to CMGs making them better suited for scanning maneuvers.
Another device, referred to as a Scissored Pair of CMGs, exists which consists of two CMGs positioned with their spin axes in the same plane but which can be individually turned so as to gimbal about two parallel axes. Scissored Pair CMGs have been proposed for use in the backpacks of persons that are used in controlling their position during space walks. CMGs used in a scissored pair can exert multiplied disturbances to the host spacecraft similarly to CMG arrays.